Previously, magnetic cores for magnetic heads, transformers and other similar devices have been manufactured by punching or stamping core chips each having a given shape with a punching press from a thin sheet ( about 0.02 to 0.1 mm thick ) of Permalloy, silicon steel or the like, magnetically annealing the chips, and laminating a given number of the chips while bonding them together with an organic adhesive. The reason why a number of chips each of a thin sheet are laminated in this way was to reduce eddy current loss and to thereby ensure favorable high-frequency characteristics.
Nevertheless, the conventional method has had the following shortcomings:
(1) With Permalloy that has been magnetically annealed ( usually in hydrogen or a vacuum kept at 1000.degree. to 1200.degree. C. for 1 to 4 hours ), even a slight strain deteriorates its magnetic characteristics. The subsequent step of lamination is liable to strain the core chips partially because of the thinness of the core chips to be laminated together, thus seriously decreasing the yield of laminated magnetic core products. PA1 (2) Laminating the punched core chips one by one requires so much time and labor that it is a major obstacle to enhancement in productivity and cost reduction in the manufacture of magnetic heads and other products.
In order to overcome these shortcomings, the present inventor previously hit upon an idea of punching out core chips at a stroke from a stack of a given number of laminated thin sheets of Permalloy or the like and then magnetically annealing the resulting laminated core chip blocks. Specifically, the inventor proposed to preliminarily laminate core thin sheets with the use of an adhesive of sodium silicate ( water glass ) prior to punching operation. This method has proved effective but showed that it causes serious problems when the laminated magnetic cores are produced in a mass production as follows.
The sodium silicate adhesive, after drying following painting for the lamination of core-forming thin sheets, would become very hard and lose its elasticity. The bonded laminate stacks thus obtained, therefore, become weaker to bending stress. For this reason, a bonded laminate stack should be fed into a punching press to be used for punching core chips in the form of a limited length. This presented a press producivity problem in the case of mass production.
Thus, it is concluded that the above proposed method is not suitable to mass production which preliminarily laminate core-forming thin sheets with the use of an adhesive of sodium silicate prior to punching operation.